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[1] The heterogeneous uptake and oxidation of SO 2 on particle surfaces representative of mineral dusts found in the atmosphere have been investigated. These particles include metal oxides (e.g., hematite, corundum), calcite, and China loess. FT-IR spectroscopy was used to characterize surface-bound species following exposure to gaseous SO 2 . It was found that SO 2 irreversibly adsorbs as sulfite (SO 3 2À ) and/or bisulfite (HSO 3 À ) on all particle surfaces with the exception of SiO 2 . The adsorbed species can be oxidized to sulfate and/or bisulfate (SO 4 2À and/or HSO 4 À ) upon exposure to ozone. Knudsen cell reactor studies were done to measure heterogeneous uptake coefficients for SO 2 . Initial uptake coefficients, calculated using BET surface areas, g BET , are found to be lower than that found for water droplets. As shown here, the g BET value for China loess can be predicted from the reactivity of the single component oxide and carbonate particles along with the average composition of the dust sample, with each component weighted by its natural abundance in the sample.

Sulfur dioxide reactions with calcium carbonate interfaces at 296 K in the presence and absence of adsorbed water result in the formation of adsorbed sulfite and sulfate. The extent of reaction is significantly enhanced, approximately five- to ten-fold for particulate and single crystal CaCO(3) (calcite), respectively, in the presence of adsorbed water between 30 and 85% RH. Atomic force microscopy following the reaction shows that adsorbed water facilitates surface reactivity by enhancing the mobility of surface ions, giving rise to the formation of nanometer sized product crystallites approximately 1 nm in height. Simultaneous with the formation of these crystallites is pitting and etching of the underlying substrate, which occurs preferentially in the vicinity of monoatomic surface steps. In the absence of water, there is little pitting and no evidence for the formation of crystallites. X-Ray photoelectron core and valence band spectra confirm the presence of two sulfur adsorbed species, SO and SO, with nearly equal amounts of SO and SO in the absence of adsorbed water and approximately five times more SO relative to SO in the presence of adsorbed water. From these data, it is proposed that the nanometer-sized crystallites are composed primarily of CaSO(3).

The uptake of ozone on various mineral powders has been studied using a Knudsen cell apparatus at T = 296 K. Samples included α‐Al2O3, α‐Fe2O3, SiO2, China loess and Saharan sand. Reactive uptake coefficients, γBET, were measured for the various particles and found to be 8 ± 5 × 10−5 for α‐Al2O3, 1.8 ± 0.7 × 10−4 for α‐Fe2O3 and 5 ± 3 × 10−5 for SiO2. The authentic dust samples were found to have γBET values of 2.7 ± 0.9 × 10−5 for China loess and 6 ± 3 × 10−5 for Saharan sand. Variations in the reactivity of the Saharan sand were observed as a function of sample treatment. Under the conditions of this study, heterogeneous uptake of ozone is catalytic and results in the destruction of ozone.

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